Two months before the fall of the Berlin Wall, Margaret Thatcher told President Gorbachev that neither Britain nor Western Europe wanted the reunification of Germany and made clear that she wanted the Soviet leader to do what he could to stop it.

In an extraordinary frank meeting with Mr Gorbachev in Moscow in 1989 — never before fully reported — Mrs Thatcher said the destabilisation of Eastern Europe and the breakdown of the Warsaw Pact were also not in the West’s interests. She noted the huge changes happening across Eastern Europe, but she insisted that the West would not push for its decommunisation. Nor would it do anything to risk the security of the Soviet Union.

Even 20 years later, her remarks are likely to cause uproar. They are all the more explosive as she admitted that what she said was quite different from the West’s public pronouncements and official Nato communiqués. She told Mr Gorbachev that he should pay no attention to these.

“We do not want a united Germany,” she said. “This would lead to a change to postwar borders, and we cannot allow that because such a development would undermine the stability of the whole international situation and could endanger our security.”

Her hardline views emerge from a remarkable cache of official Kremlin records smuggled out of Moscow. After Mr Gorbachev left office in 1991, copies of the state archives went to his personal foundation in Moscow. A few years ago Pavel Stroilov, a young writer doing research at the foundation, understood the huge historical significance of what they recorded. He copied more than 1,000 transcripts of all the Politburo discussions and brought them with him when he moved to London to continue his research.

His copies were made just in time, as all the transcripts of Politburo meetings and talks with foreign leaders have now been sealed. The records detail how the Russians reacted to the tumultuous events of 1989 and reveal the frantic attempts by Britain and France to halt moves to German unification by manoeuvring the Soviet Union into opposing it.

They also show the complete bemusement in the Kremlin in the face of riots across Eastern Europe and the flight of thousands of East Germans to Hungary and Czechoslovakia. And they make vividly clear Mr Gorbachev’s hatred of the old East European Communist leaders — he referred once to East Germany’s Erich Honecker as an “arsehole”,and his naive belief that if they were removed from office, East Europeans would be grateful to the Russians for promoting perestroika.

Mrs Thatcher knew full well that her remarks would cause a row if revealed. She was already courting controversy — especially among Solidarity supporters in Poland and the West — by telling Mr Gorbachev that she was “deeply impressed” by the courage and patriotism of General Wojciech Jaruzelski, the Polish Communist leader. She noted, approvingly, that Mr Gorbachev had reacted “calmly” to the results of the Polish elections, in which the Communists were defeated for the first time in an open vote in Eastern Europe, and to the other changes in Eastern Europe.

“My understanding of your position is the following: you welcome each country developing in its own way, on condition that the Warsaw Pact remains in place. I understand this position perfectly.”

Then she launched her bombshell. She asked that her next remarks should not be recorded. Mr Gorbachev agreed — but the Kremlin transcript included them anyway, noting laconically: “The following part of the conversation is reproduced from memory.” She spoke of her deep “concern” at what was going on in East Germany. She said “big changes” could be afoot.

And this led to her fear that it would all eventually lead to German reunification — an official goal of Western policy for more than a generation.

She assured Mr Gorbachev that President Bush also wanted to do nothing that would be seen by the Russians as a threat to their security. The same assurance was later spelt out in person to Mr Gorbachev at the Soviet- American summit off Malta.

The Kremlin records are an extraordinary snapshot of the confusion that accompanied the collapse of communism across Eastern Europe. The Russians knew that East Germany was vital to their interests, but they could no longer afford to prop it up. And Mr Gorbachev was determined not to send in troops in yet another bloody Soviet crackdown.

Amazingly, the Russians even discussed pulling down the Berlin Wall themselves, as revealed in Kremlin notes of a Poliburo discussion on November 3, 1989 — six days before the wall was opened:

[Vladimir] Kryuchkov [head of the KGB]: Tomorrow 500,000 people will come out on the streets of Berlin and other cities . . .

Gorbachev: Are you hoping that Krenz [Honecker’s replacement as party boss] will stay? We won’t be able to explain it to our people if we lose the GDR. However, we won’t be able to keep it afloat without the FRG [West Germany].

Gorbachev: They [East Germany] will be bought up whole . . . And when they reach world prices, living standards will fall immediately. The West doesn’t want German reunification but wants to use us to prevent it, to cause a clash between us and the FRG so as to rule out the possibility of a future “conspiracy” between the USSR and Germany.

Mrs Thatcher was not the only one worried by events in Germany. A month after the Berlin Wall came down, Jacques Attali, the personal adviser to President Mitterrand, met Vadim Zagladin, a senior Gorbachev aide, in Kiev.

Mr Attali said that Moscow’s refusal to intervene in East Germany had “puzzled the French leadership” and questioned whether “the USSR has made peace with the prospect of a united Germany and will not take any steps to prevent it. This has caused a fear approaching panic.”

He then stated bluntly, echoing Mrs Thatcher: “France by no means wants German reunification, although it realises that in the end it is inevitable.”

In April 1990, five months after the wall came down, Mr Attali said that the spectre of reunification was causing nightmares among France’s politicians. The documents quote him telling Mr Mitterrand that he would “fly off to live on Mars” if this happened.

Mr Gorbachev’s most difficult meetings were with the old guard in the Warsaw Pact. They were all deeply suspicion of his attempts to reform Communism. The fiercest opposition came from East Berlin.

Honecker was aged, unwell and unbending. The East German leadership feared that he was losing control and wanted to dump him. Mr Gorbachev insisted they had to sort things out themselves. Egon Krenz, Honecker’s deputy, thinking that he needed the Kremlin’s permission, had already suggested to Mr Gorbachev a coup. Three weeks later, Honecker was ousted.

Mr Gorbachev saw the chaos for himself when he went to East Berlin for the fortieth anniversary celebrations of East Germany. The entry for October 9 in the diary of Anatoli Chernayev, the Kremlin aide responsible for links with fellow Communist parties, records the tumultuous situation.

“As M.S. [Mikhail Sergeyevich Gorbachev] and Honecker walked together, a continuous roar in the air: ‘Gorby! Gorby!’ emanated from the thousands of people. Nobody paid attention to Erich . . . There were around 20 various leaders in attendance (Zhivkov, Ceaucescu, Nicaraguan Ortega etc) but nobody gave them much heed. All festivities concentrated on Gorbachev’s presence in Berlin.

“On October 10, the Socialist Unity Party of Germany will have a plenum . . . They might overthrow Erich. Otherwise it will soon come to a storm on the wall.”

Mr Chernayev noted that “all of Europe” was raving about Mr Gorbachev in Berlin. “And everybody is whispering in our ear, ‘It is good that the USSR has delicately expressed its stance against German reunification’.”

Politicians who met Mr Gorbachev’s advisers around Europe “say in unison that nobody wants a unified Germany”. Astonishingly, he noted, in France Mr Mitterrand was even thinking of a military alliance with Russia to stop it, “camouflaged as a joint use of armies to fight natural disasters”.

Mr Chernayev recorded Mr Gorbachev’s loathing of Honecker. “M.S. called him an arsehole. He said, ‘He could have said to his people that he has had four operations, he is 78, he does not have the strength to fill his position, so could they please let him go as he has done his duty. Then, maybe, he would have remained an esteemed figure in history.’ ”

If he had left two or three years earlier, he would have had a place in history, Mr Gorbachev said. Instead, Honecker was “cursed by the people”.

After the wall fell, Mr Gorbachev’s relaxed attitude to reunification hardened. At his summit with Mr Bush, he insisted that this should happen only as part of a general rapprochement in Europe. He accused the West of trying to “impose” Western values on Eastern Europe.

He also launched a ferocious attack on Helmut Kohl,the German Chancellor, for hurrying along discussion of unification. The next day, in Moscow, he accused Mr Kohl of issuing an ultimatum, of pushing unification for electoral reasons and of betraying agreements already made with Moscow.

Even in 1990 Mrs Thatcher was still trying to slow things down. “I am convinced that reunification needs a long transition period,” she told Mr Gorbachev. “All Europe is watching this not without a degree of fear, remembering very well who started the two world wars.”

It took another year of tough negotiations involving both Germanies and the four victorious wartime allies before a deal was done on unification.

Translation of the documents and additional research by Sergei Cristo.

Steps to unity

June 12, 1987 President Reagan, in a speech in front of Berlin’s Brandenburg Gate, demands: “Mr Gorbachev, tear down this wall!”

July 17, 1989 Border controls lifted between Hungary and Austria. GDR citizens flee to the West

October 7 During a visit to the GDR, Gorbachev urges reform

October 18 Erich Honecker, East Germany’s head of state, resigns. A new Government prepares a law to lift travel restrictions for East Germans going to the West

November 4 More than 500,000 people demonstrate in East Berlin, demanding reform

November 9 The Politburo announces that East Germans are allowed to move freely into West Germany. Tens of thousands flock to the Berlin Wall. Border guards with no clear orders stand aside and East Germans stream through

November 10 The Brandenburg Gate is opened

May 18, 1990 The two German states sign a treaty on monetary, economic and social union, which comes into force on July 1

October 3 East Germany joins the Federal Republic of Germany Source: German Embassy and Times database